All bicycles have points of attachment where the rear wheel axle is affixed to the frame. These attachment points are commonly referred to as “dropouts,” because they permit the rear wheel to drop from the frame after the axle attachment hardware is loosened. The present disclosure uses the term “dropout” to generally refer to the various types of rear axle attachment structures of a bicycle frame.
Bicycle hubs have axles that typically extend into axle slots on the left and right dropouts. A lock nut or other shoulder near each end of the axle contacts the inner surface of the dropouts and locates the hub laterally between the dropouts. The hub is attached to the dropouts by exerting a clamping force between the axle shoulder and outside of the dropout, through the use of nuts, bolts or a quick release device. Common dropout configurations are the horizontal dropout, vertical dropout, semi-vertical dropout and track dropout. The distance between the locknuts or shoulders on the axle is commonly referred to as the “over locknut dimension.” The distance between the inner surfaces of the dropouts typically limits the over lock locknut dimension that may be used with a particular frame.
Bicycle frames are typically designed and manufactured to establish a fixed distance between the inner surface of the dropouts and will accommodate only hubs having a specified over locknut dimension. Other wheel sizes, axle lengths and drivetrain configurations may not be compatible with one particular bicycle frame. Although steel frames can be bent or cold set to change dropout spacing, this can result in visible bowing of the chainstays and seatstays and can weaken the dropouts. Bending a frame requires careful realignment, is time consuming and can be expensive. Frames constructed of aluminum or carbon fiber cannot be bent to change dropout spacing.
In addition to a particular frame setting a fixed distance between dropouts, the design of the dropouts can also limit the components, such as hubs, drivetrains, brakes, and the like, that can be used with the frame. Most road frames are constructed with vertical dropouts intended for use with derailleurs. Vertical dropouts are incompatible with fixed-gear drivetrains because the axle cannot be moved forward or backward to adjust tension on the chain. In another example, frames with rear-opening track style dropouts are ill-suited for use with derailleur gears, because the derailleur inhibits the removal of the wheel.
Hence, a need exists for dropout assemblies that can accommodate various drivetrain types and axles with different over locknut dimensions on a single frame.